President Trump ramped up his criticism of Amazon Friday, tweeting that the e-commerce giant should be socked with sharply higher shipping charges by the U.S. Postal Service.

"Why is the United States Post Office, which is losing many billions of dollars a year, while charging Amazon and others so little to deliver their packages, making Amazon richer and the Post Office dumber and poorer? Should be charging MUCH MORE!" Trump said in the early-morning tweet during a Christmas season vacation at his Mar-a-Lago estate in Florida.

Why is the United States Post Office, which is losing many billions of dollars a year, while charging Amazon and others so little to deliver their packages, making Amazon richer and the Post Office dumber and poorer? Should be charging MUCH MORE!

The tweet is the latest in a series of critical tweets that Trump has directed at Amazon, whose founder and CEO, Jeff Bezos, also owns The Washington Post— the national news organization that has broken many news stories that spotlighted shortcomings involving the president and his White House administration.

For instance, Trump in July tweeted that the "Amazon Washington Post" fabricated information about his decision to end U.S. payments to rebels fighting the forces of Syrian President Bashar Assad.

Trump's new tweet on Friday at least temporarily shifted Amazon shares from positive to negative. The company's stock finished the day 1.4% lower at $1,169.47 a share but ended up more than 55% for the year.

Representatives of Amazon and the postal service did not immediately respond to emails seeking comment.

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U.S. Postal service mail handler Barbara Lynn sorts boxes at the Royal Palm Processing and Distribution Center in Opa Locka, Fla. The U.S. Postal Service is projecting more than 850 million packages will be sent during the holiday shopping and gift giving season, which is more than a 10 percent increase compared to the same period last year. Joe Raedle, Getty Images

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After reporting a surplus in 2006, the postal service has reported net losses during each of the 11 succeeding fiscal years. The 2017 net loss totaled $2.7 billion, the postal service reported in November. Most of the losses resulted from changes in interest rates, "outside of management's control" that reduced workers' compensation costs.

Earlier in December, the Postal Regulatory Authority moved to stem the red ink by allowing the postal service to raise the cost of a first-class stamp, currently 49 cents, by 2% above the rate of inflation. The move eventually is expected to raise shipping costs for Amazon and other companies, as well as for magazines, prescriptions, and other mail.

The postal service's data shows its volume of first-class single-piece mail, consisting of bill payments, letters, personal correspondence, and cards, totaled 19.7 billion last year. That represents a 4.8% decline from 2015, and less than half of the 42.3 billion volume handled in 2007.

However, total shipping/package volume rose to 5.2 billion last year, a nearly 16% increase over 2015, the postal service data shows.

The Postal Accountability and Enforcement Act enacted in 2006 blocks the postal service from using its essential monopoly over letter deliveries to subsidize lower costs for package shipping that could hurt private shippers like UPS and FedEx.

In contrast to the postal service, Seattle-based Amazon has become a global retail giant, shipping books, clothing and a broad array of other products to consumers who increasingly prefer to do their shopping online. Amazon's stock for the first time topped $1,000 a share in April, and have continued to climb.

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President Trump’s twitter finger’s just lost retailer, Amazon 6 billion dollars. Veuer's Natasha Abellard (@Natasha Abellard) has the story
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Shipping expenses represent a major expense for Amazon. Although the company sends some deliveries through the U.S. Postal Service, it also relies on UPS and Federal Express for shipping.

FedEx and UPS shares fell in October amid news reports that Amazon was testing its own delivery system to whisk packages from the company's warehouses to consumers' homes.